A honeycomb structure is widely used for catalyst carrier, various types of filter, and the like. Recently, the honeycomb structure attracts attention as a diesel particulate filter (DPF) for trapping particulate matter emitted from diesel engines.
Generally, a principal component of the honeycomb structure is ceramics in many cases. To fabricate such a honeycomb structure, firstly water and various additives such as binder are added to raw material of ceramics to prepare kneaded clay, then a formed article with a shape of honeycomb (honeycomb formed article) is made through extrusion forming. After drying the honeycomb formed article, this honeycomb formed article is fired and then fabrication of the honeycomb structure can be achieved.
As drying methods of the honeycomb formed article: an dielectric drying method, which uses high frequency energy generated by current between electrodes provided upper and lower part of the honeycomb formed article; and a hot air drying method, which performs drying through introducing hot air generated by gas burner and the like, are well known. However in these days, in place of or in addition to these drying methods, a drying method utilizing microwaves (microwaves drying method), which has advantages of quick drying speed and the like (for example, refer to Patent Documents 1 to 3) has been adopted.
However, such microwave drying method has had difficulty in drying the whole honeycomb formed article in a uniform speed, due to delayed drying in the upper and lower end portion or in the peripheral portion of the honeycomb formed article compared with other portion in drying process. The honeycomb formed article shrinks when water evaporates whereby when drying speed is not uniform, defects such as deformation and breakage tend to happen easily. Moreover, thinning of partition wall (rib) to separate cells has been progressed and the thinner the partition wall of the honeycomb formed article is, the more easily deformation of the honeycomb formed article occurs. Consequently, uniformalizing of drying speed has especially become to be an important object recently.
Patent Document 1: JP-A 2002-283329
Patent Document 2: JP-A 2002-283330
Patent Document 3: WO 2005/023503 Pamphlet